Different molar ratios of 90Y-labeled r-BHD toward Av were studied. The same amount of radiolabeled r-BHD (1 µg) was reacted with decreasing amounts of Av (Mw 66 kDa), to obtain 1:2, 1:4, 1:8, 1:16, and 1:32 Av/rBHD molar ratios. The mixtures were incubated at 37 °C and analyzed at 2 and 24 h using Centricon YM-10 concentrators (Millipore, Bedford, MA, 10kDa Mw cutoff). Aliquots from the top and bottom parts of the Centricon were counted in the â-counter as previously described. To evaluate the activity a specifically bound to the Centricon, an 90Y-labeled r-BHD aliquot was centrifuged without adding Av.